1. Field of the Invention
This invention relates generally to the rolling of long products, and is concerned in particular with an improved method and apparatus for continuously hot rolling ferrous rods and bars.
2. Description of the Prior Art
In the conventional steel rod rolling mill, as depicted schematically in FIG. 1, a plurality of roll stands S1-S27 are aligned along a rolling line to continuously roll billets received from a furnace 10 or other like source. The roll stands are arranged in successive groups which typically include a roughing group 12, an intermediate group 14 and a finishing group 16. The roll stands of the roughing and intermediate groups are usually individually driven, and are arranged alternately with horizontal and vertical work rolls, or in some cases with housings that can be adjusted to achieve either horizontal or vertical work roll configurations.
The roll stands of the finishing group 16 are usually mechanically connected to each other and to a common drive to provide an arrangement referred to as a "block" (illustrated diagrammatically at 18 in FIG. 1). U.S. Pat. Nos. Re.28,107 and 4,537 055 provide illustrative examples of blocks well known and widely employed throughout the metals industry. The mill rolling schedule will usually be based on an oval-round pass sequence, with guides being arranged between the roll stands to direct the product from one roll pass to the next along the rolling line.
Modern mills of the above-described type must have the capability of meeting diverse and increasingly demanding customer requirements, not the least important of which is the ability to supply a wide range of product sizes. For example, a rod mill should ideally be capable of supplying round rods ranging from about 3.5 to 25.5 mm in diameter.
When changing from one product size to another, the mill must be shut down in order to afford operating personnel an opportunity to make the necessary adjustments to the rolling equipment. Such adjustments include changing work rolls and guides, rendering selected stands inoperative by either removing them from the rolling line or removing their work rolls (a practice commonly referred to as "dummying"), etc.
The duration and frequency of such shutdowns can have a severe negative impact on overall mill utilization. For example, in the conventional mill illustrated in FIG. 1, even when making a relatively modest change from rolling a family of products having as its smallest size a 5.5 mm diameter round to another family of products having as its smallest size a 6.0 mm round, the work rolls of the roll passes in stands S12 to S19 of the intermediate mill 14 and all of the work rolls in stands S20 to S27 of the block 18 must be changed. In addition, most if not all of the guides between stands S12 to S29 also must be changed. This can take up to an hour to complete, at a significant loss in production time and profit to the mill owner.
Because of this, mill operators are reluctant to frequently make major changes to product sizes, preferring instead to roll the same or closely related sizes within the same family for protracted periods. This not only increases product storage requirements and inventory costs, but also fails to provide the flexibility often needed to meet customer requirements. The need to store a wide variety of work rolls and guides further exacerbates inventory costs.
There is also a growing demand to have products "sized", i.e., finish rolled to extremely close tolerances on the order of those approaching cold drawn tolerances. The tolerances achieved through sizing enable products to be employed "as rolled", i.e., without having to be additionally subjected to expensive machining operations such as "peeling" or "broaching". Such high tolerance products are required, for example, in the manufacture of bearing cages, automotive valve springs, etc. Also, depending on the type of steel being processed and the intended end use of the product, the customer may further require that finish rolling be carried out at temperatures at or about the A.sub.3 temperature (a process which can be classified as "thermomechanical rolling"). Thermomechanically rolled products rolled below the recrystalization temperature retain a flattened or "pancaked" fine grain structure which increases tensile strength while at the same time shortening the time required for subsequent heat treatments, e.g., spheroidized annealing.
In the conventional sizing operation, the product exiting from the last stand of the finishing group 18 is subjected to further rolling in so-called "sizing" stands. The sizing stands achieve the desired close tolerances by affecting relatively light reductions in a round-round pass sequence. A recent development in sizing technology as it relates to larger diameter bar products is disclosed in U.S. Pat. No 4,907,438 issued Mar. 13 1990 to Sasaki et al. Here, the sizing stands are grouped in block form at a location downstream from the delivery end of the finishing section of a bar mill. The sizing stands have fixed interstand drive speed ratios and a round-round pass sequence adapted to take relatively light reductions on the order of 8.7-13.5%. By changing groove configurations and/or roll partings in the roll stands of the sizing mill, and by dummying out selected upstream roll stands in the intermediate and/or finishing mill sections, it is theoretically possible to produce an incremental range of finished product sizes, thereby improving operating efficiency and mill utilization.
However, experience has indicated that such improvements may be offset and in some cases put entirely out of reach by the development in certain products of a duplex microstructure, where the grains throughout the cross-section of the product vary in size by more than about 2 ASTM grain size numbers*. This phenomenon, commonly referred to as "abnormal grain growth", is particularly pronounced in medium carbon and case hardening steel grades. FNT * Measured in accordance with ASTM E112-84.
It is generally recognized that a variation of more than about 2 ASTM grain size numbers in the cross-section of a product can cause rupturing and surface tearing when the product is subjected to subsequent cold drawing operations. Such grain size variations also contribute to poor annealed properties, which in turn adversely affect cold deformation processes.
It has now been determined that abnormal grain growth can occur as a result of the time interval which conventionally occurs between the last significant reduction which takes place during normal rolling and the lighter reductions which take place during sizing.
More particularly, in the roll stands of the roughing, intermediate and finishing groups, the product is subjected to relatively high levels of successive reductions on the order of 15 to 30%. Each such reduction produces an increased energy level in the product sufficient to create a substantially uniform distribution of fine grains. Depending on time, temperature and chemical composition, after each sequential reduction the internal energy produced by deformation instantly begins to dissipate by recovery, recrystallization and grain growth. At each successive significant reduction, the increased internal energy state is reestablished, which again refines the microstructure. Thus, as the product proceeds through the mill and is rapidly subjected to relatively high levels of successive reductions, it retains a substantially uniform fine grained microstructure.
However, after the last significant reduction, grain growth again commences. The extent to which grain growth continues is directly dependent on time, temperature and the chemical composition of the steel being rolled. The relatively light reductions which are taken subsequently in the sizing stands are insufficient to affect the entire microstructure of the product, since only grains at the product surface are deformed.
Thus, unless sizing occurs sufficiently soon after the last significant mill reduction, the intervening unabated grain growth coupled with only localized surface grain deformation during sizing will produce an unacceptable dual grain microstructure, with the size of grains varying significantly throughout the cross-section of the product.
This phenomenon is further illustrated in FIGS. 2A and 2B. FIG. 2A includes photomicrographs (X150) showing the grain structure at selected locations in the cross-section of a 12.5 mm rod, steel grade 1040, with uniform grain structure prior to sizing. FIG. 2B includes photomicrographs at the same magnification of the same rod after it has been subjected to a 7.6% reduction in two round sizing passes. The resulting duplex microstructure is plainly evident.
As the rolling schedule changes and stands are progressively dummied back through the finishing and intermediate sections of the mill in order to feed the sizing stands with progressively larger products, the time interval between the last significant reduction and the commencement of sizing increases, thereby exacerbating the abnormal grain growth problem.
Some attempts have been made at eliminating duplex microstructures by taking higher reductions in the round passes of the sizing stands. While this practice does yield more uniform microstructures, it does so at the cost of poorer tolerances and a marked decrease in the ability of the mill to roll a range of product sizes without changing roll grooves (a practice commonly referred to as "free size rolling").
The fixed interstand drive speed ratios of conventional sizing stands also seriously limit the possibility of combining sizing with other operations, e.g., thermomechanical rolling.